"The goal of an incremental backup is to back up only those data
blocks that have changed since a previous backup".

So, If backup has been taken at level 0 then incremental backup
will be done at level 1, means backup will be done only for
those data which has been changed after 0 level backup.

Now let's undertstand the diffrenece between Differential Backup
and Cumulative backup:

"A differential backup, which backs up all blocks changed after
the most recent incremental backup at level 1 or 0."
RMAN determines which level backup occurred most recently and
backs up all blocks modified after that backup. If no level 1 is
available, RMAN copies all blocks changed since the level 0
backup.
If no level 0 backup is available, then the behavior depends upon
the compatibility mode setting. If compatibility is >=10.0.0,
RMAN copies all blocks changed since the file was created, and
stores the results as a level 1 backup. In other words, the SCN
at the time the incremental backup is taken is the file creation
SCN. If compatibility <10.0.0, RMAN generates a level 0 backup,
to be consistent with the behavior in previous releases.
|<------------
| <------
| <------
| <------
| <------
| <------
|<------
|<------------|
| <------ |
| <------ |
| <------ |
| <------ |
| <------ |
|<------ |
0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 0
S M T W T F S S S M T W T F S S
In the example shown in Figure above, the following occurs:
Sunday :
An incremental level 0 backup backs up all blocks that have ever
been in use in this database.
Monday - Saturday
On each day from Monday through Saturday, a differential
incremental level 1 backup backs up all blocks that have changed
since the most recent incremental backup at level 1 or 0. So,
the Monday backup copies blocks changed since Sunday level 0
backup, the Tuesday backup copies blocks changed since the
Monday level 1 backup, and so forth.
The cycle is repeated for the next week

"A cumulative backup, which backs up all blocks changed after the
most recent incremental backup at level 0"
In a cumulative level 1 backup, RMAN backs up all the blocks used
since the most recent level 0 incremental backup. Cumulative
incremental backups reduce the work needed for a restore by
ensuring that you only need one incremental backup from any
particular level. Cumulative backups require more space and time
than differential backups, however, because they duplicate the
work done by previous backups at the same level.

In the example shown in above fig, the following occurs:
Sunday :
An incremental level 0 backup backs up all blocks that have ever
been in use in this database.
Monday - Saturday
A cumulative incremental level 1 backup copies all blocks changed
since the most recent level 0 backup. Because the most recent
level 0 backup was created on Sunday, the level 1 backup on each
day Monday through Saturday backs up all blocks changed since
the Sunday backup.
The cycle is repeated for the next week.

The point is that if you are implementing Differential one. you will require all the n level backup during recovery.
While in the cumulative one. taking a fresh level n backup makes the other level n backup redundant. (you require only copy during recovery, as it alone has all the changes that has happened from the last level n-1 backup)

Copyright 1998-2015 Ziff Davis, LLC (Toolbox.com). All rights reserved. All product names are trademarks of their respective companies. Toolbox.com is not
affiliated with or endorsed by any company listed at this site.